Reliability indices of an electric distribution system, especially the System Average Interruption Duration Index (SAIDI) and Customer Average Interruption Duration Index (CAIDI), are one the major measures of power utility performance. The SAIDI is generally defined as the average outage duration for each utility customer served. The CAIDI is a measure of the average customer outage time, other wise known as an average restoration time. These performance indicators are often tied to rate increases or other monetary incentives for power utilities. Thus, it is imperative that power utilities minimize the scope and duration of power outages to maintain optimal SAIDI and CAIDI values.
One method of maintaining high network reliability is by properly maintaining the physical system components such as transformers, circuit breakers, overhead lines, etc. The other major contributor to network reliability indices is fault response. Specifically, it is important to quickly restore service to load zones that are not at fault, but experience a power interruption because of a fault elsewhere on the power network.
In many utility network grids, fault response is administered by an outage management system (OMS). These OMS systems locate faulted zones using customer phone calls using sophisticated tracing algorithms. The OMS systems then rely on dispatchers to execute a restoration switching analysis (RSA) to determine a service restoration switching plan. A service restoration switching plan includes one or more network switching instructions which, when performed, restores power to one or more load zones. The switch open/close commands may be executed automatically/electronically, through a supervisory control and data acquisition (SCADA) system or by communications between dispatchers and mobile ground crews.
The SCADA and OMS systems are part of the distribution management system (DMS). The response time of the DMS to an actual outage is determined by many factors, including the number and location of tele-metered components in the field, the number and location of remote-controllable switching devices in the fields, the number and location of dispatchable crews on the ground. Perhaps the most important response time variable is how an outage is determined in the DMS and how the switching operations are executed. Because human interaction is required at each step of the process, the restoration time is often at least several minutes or more. By involving human interaction, fault locating and restoration may be more nuanced, but overall restoration time reduction is negatively impacted.
The use of automatic reclosers greatly improves the reliability of the overhead distribution system by eliminating the need of human involvement in case of temporary/intermittent faults. Reclosers, upon sensing a fault, open much like a circuit breaker. After a predetermined amount of time, the recloser attempts to close. This process is repeated until the fault is cleared or until a preset number of attempted closing events are counted, at which time the recloser remains open. In this manner, many intermittent faults that would ordinarily cause a power outage are cleared without permanent loss of power. However, reclosers cannot eliminate the need for back-feeding power restoration in case of permanent faults.
Smart protection and control relays, or intelligent electronic devices (IED), have been used to execute back-feeding switching controls though peer-to-peer or other communication methods. However, the technology can only be employed in simple distribution systems where back-feeding source capacity, back-feeding feeder loading capability, and other circuit constraints (such as voltage violation) are of minimal concern. Existing overhead distribution systems, especially relatively older networks, are often much more complicated than these simple distribution system models.
It is therefore advantageous to provide an automated system that is capable of performing RSA and restoration functions in more complex networks.